From EP-A 100 119 it is known that propene can be converted by hydrogen peroxide into propene oxide if a titanium-containing zeolite is used as catalyst.
Unreacted hydrogen peroxide cannot be recovered economically from the epoxidation reaction mixture. Furthermore, unreacted hydrogen peroxide involves additional effort and expenditure in the working up of the reaction mixture. The epoxidation of olefin is therefore preferably carried out with an excess of olefin and up to a high hydrogen peroxide conversion. In order to achieve a high hydrogen peroxide conversion it is advantageous to use a continuous flow reaction system. Furthermore, high selectivity for the desired olefin oxide is important for an industrial scale process to achieve high yields and to reduce costs for subsequent work-up.
However, activity and selectivity of the above described titanium-containing zeolite catalysts are sharply reduced over time in a continuous process for the epoxidation of olefins, making frequent catalyst regeneration necessary. For an industrial scale process, this is not acceptable for economic reasons.
In the literature numerous routes to either increase catalyst activity and/or selectivity or to reduce catalyst deactivation for the above described titanium-containing zeolite catalysts are described:
For example, from EP-A 230 949, it is known to neutralize the titanium silicalite catalyst either prior to its use in an epoxidation reaction or in situ with strong bases thereby introducing large amounts of alkali metal or alkaline-earth metal ions into the reaction mixture. This neutralization resulted in an increase in activity and selectivity to form the desired olefin oxide in a batch process.
The experiments in EP-A 757 043, however, show that in a continuous process the activity is considerably reduced, if the catalyst is neutralized prior to or during the reaction. Therefore, it is suggested to treat the catalyst prior to or during the epoxidation reaction with a neutral or acidic salt. The experimental data in EP-A 757 043 confirm that by addition of neutral or acidic salts the selectivity is increased but the activity is less reduced compared to the addition of a base. But EP-A 757 043 only shows examples wherein the catalyst is treated with the salt prior to the reaction and the catalyst is used in slurry form. Additionally the experiments were only run for 8 hours but nevertheless show a dramatic drop in catalyst activity only after 4 hours which is by no means acceptable for an industrial process.
Similarly EP-A 712 852 teaches that by performing an epoxidation process catalyzed by titanium silicalite in the presence of a non-basic salt the selectivity is increases. All the examples are run in batch operation mode with a stirred catalyst slurry for one hour. Although it can be confirmed that the presence of non-basic salts may have a positive influence on catalyst selectivity in a short term experiment, it was discovered that even if non-basic salts are present in a reaction mixture for a continuous epoxidation reaction the activity and selectivity drops dramatically over time. Thus the teaching of EP-A 712 852 does not lead to a reaction system that can be economically employed in a continuous epoxidation process using hydrogen peroxide in presence of a heterogeneous catalyst.
Several patent documents deal with the problem of optimizing activity and selectivity of titanium silicalite catalyst in epoxidation reactions by means of addition of nitrogen containing compounds and pH-adjustment of the reaction mixture. For example EP-A 1 072 599 teaches the addition of nitrogen containing bases to the reaction mixture, whereas EP-A 1 072 600 discloses the use of a buffer system comprising salts of those nitrogen containing bases for pH adjustment. EP-A 940 393 relates to the addition of amide group containing organic compounds to the epoxidation reaction mixture. U.S. Pat. No. 6,429,322 discloses the addition of strong bases like alkali or alkaline earth metal or tetraalkyl ammonium hydroxide and the addition of weak bases like ammonium hydroxide or alkali or alkaline earth metal salts of weak acids for pH adjustment of the reaction mixture. But in none of these references is the effect of impurities commonly present in commercially available aqueous hydrogen peroxide solution on the long term activity and selectivity of the titanium silicalite catalyst addressed or investigated.
Today, the vast majority of hydrogen peroxide is produced by the well-known anthraquinone process. A survey of the anthraquinone process and its numerous modifications is given in G. Goor, J. Glenneberg, S. Jacobi: “Hydrogen Peroxide” Ullmann's Encyclopedia of Industrial Chemistry, Electronic Release, 6th ed. Wiley-VCH, Weinheim Jun. 2000, page 14.
Crude hydrogen peroxide solutions or concentrated hydrogen peroxide solutions obtained from the anthraquinone process contain a plurality of compounds in addition to hydrogen peroxide in low concentrations. These compounds are either impurities or additives like stabilizers. The impurities are compounds that are extracted from the working solution into the aqueous phase. They are mainly ionic or polar species like carboxylic acids, alcohols, carbonyl compounds and amines. These impurities are therefore also found in commercial hydrogen peroxide solutions.
For example, hydroquinone solvents that are commonly used in the above described process are nitrogen containing compounds like amides and ureas (see Ullmann, supra, page 6). Particularly preferred are tetraalkyl ureas like tetrabutyl urea. The use of these solvents result in amine impurities like monoalkyl or dialkyl, especially monobutyl and dibutyl, amines in the final hydrogen peroxide solutions. For example the commercial hydrogen peroxide solution HYPROX® available from Degussa AG contains up to 200 wppm mono- and dibutyl amine based on the weight of hydrogen peroxide.
In WO 00/76989 the influence of ionic components in commercially available aqueous hydrogen peroxide solutions that are used in epoxidation reactions as described in the above prior art documents is discussed. Ionic components, especially phosphates and nitrates, are added to commercially available aqueous hydrogen peroxide solutions as stabilizers to reduce hazardous decomposition of hydrogen peroxide. WO 00/76989 teaches contrary to the disclosure in the above prior art documents that the presence of ionic components in the reaction mixture, even those that have been added as stabilizers to commercial hydrogen peroxide, are detrimental to the long term selectivity in a continuous titanium silicalite catalyzed epoxidation reaction and should therefore be reduced to a minimum. Contrary to the above prior art documents, continuous reactions running up to 300 hours were conducted showing that if ionic components are present in an amount of more than 100 ppm the long term selectivity is reduced. To solve this problem, it is suggested to remove ionic components from hydrogen peroxide solutions prior to use in epoxidation reactions by means of ion exchangers. Moreover, WO 00/76989 teaches that ammonium compounds and ammonia should be avoided under any circumstances since these compounds may lead to undesired side products by oxirane ring opening reactions with the formed olefin oxide. Although the teaching in WO 00/76989 leads to some improvement in long term selectivity compared to the above prior publication, this improvement is still insufficient for an industrial scale process. Furthermore, this improvement can be achieved only with the complicated and, both in terms of investment and process costs, economically undesirable additional process step of ion exchange. Last but not least, removal of stabilizing ions like phosphate and nitrate from the hydrogen peroxide solution makes the process more hazardous and additional measures have to be taken to ensure safety during the entire process.
Contradicting the teaching of WO 00/76989, WO 01/57012 discloses that the use of crude hydrogen peroxide solutions directly obtained from the anthraquinone process having large amounts of, for example, sodium, nitrate, phosphate, and organic impurities, is superior with respect to product selectivity compared to highly purified hydrogen peroxide solutions comprising very low amounts of sodium, nitrate, and phosphate. The experiments, however, were only conducted for a few hours so that the long term activity and selectivity of the catalyst cannot be determined from that reference.
Again, another approach is shown in WO 01/92242, wherein a titanium silicalite catalyzed process for epoxidation of olefins using crude hydrogen peroxide solutions in the presence of a compound having aminocarbonyl functionality in which the nitrogen bears at least one hydrogen atom is disclosed. The examples show a batch type process that is conducted up to a conversion of hydrogen peroxide of 85%. After two hours, the reaction is terminated even if the conversion of 85% has not been reached. Although the experimental data show an improvement with respect to the reaction rate compared to compounds with aminocarbonyl functionality having no hydrogen atom bonded to the nitrogen atom long term activity and selectivity of the catalyst in a continuous process is not determinable from the information in WO 01/92242.
DE-A 199 36 547 discloses a continuous titanium silicalite catalyzed process for epoxidation of olefins with hydrogen peroxide whereby the conversion is kept constant by increase of reaction temperature and adjusting the pH of the reaction mixture. In a long term experiment (1000 hours), it could be verified that by adjusting the pH the increase in temperature and the rate of increase could be reduced compared to an experiment without pH adjustment. But conversion and selectivity were the same irrespective whether the pH was adjusted or not.
Thus, an object of the present invention is to provide a continuous process for the epoxidation of olefins with hydrogen peroxide in the presence of a heterogeneous catalyst promoting the epoxidation reaction whereby an improvement in long term activity and selectivity of the catalyst compared to the above discussed prior art can be achieved without adding additional process steps and in an economic way.